1. Field of the Invention
The present invention relates to an organic electroluminescence element which is preferably used in displays, light emitting elements, and the like in various devices of the information industry. In particular, the present invention is directed to an anode structure of an organic electroluminescence element.
2. Description of the Related Art
Prior to the present invention, an electroluminescence element (hereinafter, "organic EL element") was used as an electric field light emitting element in units or pixels in display devices of various industrial instruments. FIG. 4 shows an outline sectional view of essential portions of such a conventional organic EL element. As shown in FIG. 4, an organic EL element 100 comprises four components: a glass substrate 101, an anode 102, an organic layer 103, and a cathode 104. See FIG. 4. The transparent anode 102 is formed on the surface of the transparent glass substrate 101, and the organic layer 103, comprising an organic fluorescent thin film or an organic hole transport layer or the like, is formed on the anode 102. Finally, the cathode 104, constructed from a metal, is formed on the organic layer 103 by vacuum evaporation or the like.
When the organic EL element 100 is designed to emit light, a drive source 105 is connected between the cathode 104 and the anode 102. See FIG. 4. A current flows through the organic layer 103 by a voltage supplied from the connected drive source 105, resulting in the organic layer 103 emitting light. The emitted light is emanated to the outside of the organic EL element via the transparent glass substrate 101.
There are several problems observed with conventional organic EL elements. Generally, a transparent electrode, referred to as an Indium-Tin-Oxide ("ITO"), is used as the anode of the organic EL element. The ITO is formed on the transparent glass substrate by sputtering, electron beam evaporation ("EB"), or the like. However, with the use of such formation methods, the resultant surface of the formed ITO is rough and irregular. The surface roughness of such an ITO as measured by the maximum height ("R max") of the surface roughness as defined in the definition and description (B0601) of the surface roughness prescribed by the Japanese Industrial Standards ("JIS") is in the order of several tens to several hundreds of Angstroms. The maximum height or R.sub.max is defined and measured according to JIS No. B0601 in the following manner. A reference length along the surface to be measured is taken along a mean line along the surface of the anode. The mean line is drawn in a direction parallel to the longitudinal axis of the anode. Preferably, the reference length is approximately 20% of the length of the entire surface being evaluated. On the surface of the anode, peaks extend above the mean line and valleys extend below the mean line. First, the highest peak and the lowest valley along the reference length are determined. Second, the distance between the top of the highest peak and the mean line, as measured perpendicularly to the mean line, is determined. Third, the distance from the mean line to the bottom of the lowest valley, as measured perpendicularly to the mean line, is determined. These distances are summed and the total is the maximum height, R.sub.max. While determining the maximum height, the reference length shall be sampled from the anode face along an area which is free from extraordinary high peaks and deep valleys that may be considered to be flaws.
The roughness of the ITO element results in the generation of leakage current, which results in less light being emitted from the element. Specifically, the thickness of the organic layer 103 laminated on the anode 102 of the organic EL element 100 is very thin--approximately 1000 to 2000 Angstroms. Because the lamination of the organic layer 103 is thin, projections of the rough surface of the ITO create varying distances between the anode and the cathode; i.e., when projections are present on the surface of the ITO, the distance between the anode and the cathode at the projected portions is less than at other portions on the ITO. As a result, when a voltage flowing in a positive direction (which causes the element to emit light) is applied on the surface-rough ITO element, current flows in a concentrated manner on the projected portions of the ITO. This phenomenon is referred to as leakage current. Leakage current is generated when the element is emitting light, the current-brightness characteristic is deteriorated as less than 100% of the current is being utilized to generate light. That is, the anode and the cathode are short-circuited at the projected portions: the current flows only at the projected portions and as a result, the element ceases to emit light.
Moreover, conventional organic EL elements also exhibit inverse bias current, which results in decreased efficiency of the element. Specifically, when leakage current is generated in an organic EL element, the current is apt to flow in an inverse direction at the projected portions. This results in a phenomenon in which an inverse bias current, or a current resulting from application of the voltage in the inverse direction, is increased, producing an unstable current value. The deterioration of the inverse bias voltage caused by the leakage current results in a deterioration of the efficiency of the element.